The present application claims priority to Japanese Application No. P11-161253 filed Jun. 8, 1999 and Japanese Application No. P11-161252 filed Jun. 8, 1999, which applications are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to an ultraviolet optical device, e.g., an ultraviolet ray generation device, or an ultraviolet optical device such as an exposure device using an ultraviolet ray.
2. Description of the Prior Art
Conventionally, an ultraviolet ray having a wavelength of 400 nm or less is irradiated on an optical part in the atmosphere. Moisture and oil from the atmosphere that are on the surface of the optical part react, and reactants, peripheral particles, and the like adhere to the surface of the optical part. As a result, the optical characteristics of the optical part are inconveniently deteriorated.
Particularly, in waveform conversion (M. Oka and S. Kubota, JJAP. Vol. 31 (1992) pp 513 or M. Oka et. al., of Conference on Laser and Electro-Optics (OSA, Washington D.C., 1992) or the like of an external resonator type, a small deterioration in performance of a mirror or a nonlinear optical element arranged in a resonator considerably reduces a harmonic output which is generated.
For example, an ultraviolet optical device of an external resonator type, which converts, e.g., a fundamental wave having a wavelength of 532 nm into an ultraviolet ray having a wavelength of 266 nm in a resonator, has first to fourth mirrors 1 to 4 constituting an external resonator as shown in FIG. 20. The first to third mirrors 1 to 3 are constituted by high-reflectance mirrors each having a reflectance of, e.g., 99.95% or more with respect to the wavelength of 532 nm of the fundamental wave, and the fourth mirror 4 is constituted by a mirror having a reflectance of, e.g., 99% or more with respect to the wavelength of 532 nm.
Between the first and fourth mirrors 1 and 4, a nonlinear optical element 5, e.g., barium borate xcex2-BaB2O4 (to be referred to as BBO hereinafter) is arranged. Both the end faces, i.e., a light-incidence end face 5a and a light-emission end face 5b of the nonlinear optical element 5 are polished into mirror planes, respectively, to form low-reflectance films each having a reflectance of 0.1% or less with respect to the wavelength of 532 nm of the fundamental wave.
With this configuration, incident light 6, i.e., a fundamental wave having the wavelength of 532 nm and reaching through the fourth mirror 4, is amplified among the first to fourth mirrors 1 to 4, and an ultraviolet ray having a wavelength of 266 nm of the second harmonic wave of the fundamental wave is led by the nonlinear optical element 5 from the light-emission end face 5b. An ultraviolet ray obtained by waveform conversion as described above is output as emission light 7 through the first mirror having a transmittance which is high with respect to the wavelength of the ultraviolet ray.
In the external resonator, the third mirror 3 is arranged in an actuator (not shown) constituted by, e.g., a so-called VCM (Voice Coil Motor), and the position of the third mirror 3 is adjusted.
When the wavelength conversion is performed in the atmosphere, the optical characteristics of the mirrors constituting the external resonator deteriorate. More specifically, an optical loss caused by enlargement of scattering increases. In particular, on the light emission end face side of the nonlinear optical element, which receives large quantity of ultraviolet light, i.e., light having a short wavelength of 400 nm or less, an optical loss on the first mirror 1 in FIG. 20 considerably increases.
This optical loss and an output Pxcfx89 obtained by amplifying the fundamental wave having a wavelength of 532 nm in the external resonator are expressed by the following equation (11):
Pxcfx89=(xcex4cav2+4xcex3SHPixe2x88x92xcex4cav)/2xcex3SHxe2x80x83xe2x80x83(11)
where, xcex4cav is an optical loss at a wavelength of 532 nm in the external resonator, Pxcfx89 is an output of an amplified fundamental wave, Pi is an output of an incident fundamental wave on to the nonlinear optical element 5, and xcex3SH is a constant which is called a nonlinear conversion factor determined on the basis of the crystal length of the nonlinear optical element 5, the wave length of the fundamental wave, the spot size of the incident fundamental wave, and a focusing parameter.
According to equation (11), it is understood that, when the optical loss xcex4cav increases, the output Pxcfx89 of the fundamental wave decreases.
On the other hand, the relationship between the output Pxcfx89 of the fundamental wave and an output P2xcfx89 of a second harmonic wave can be expressed by the following equation (12):
P2xcfx89=xcex3SHPxcfx892xe2x80x83xe2x80x83(12)
According to equation (12), it is understood that, when the output Pxcfx89 of the fundamental wave decreases, the output P2xcfx89 of the second harmonic wave also decreases.
Actually, with the configuration in FIG. 20, when a secular change in output was measured, where ultraviolet light was generated in the atmosphere in a clean room (volume fraction of moisture amount of about 8,000 [ppm]), the results shown in FIG. 18 were obtained.
As is apparent from the results, an ultraviolet output becomes half about 47 hours after in the atmosphere having the moisture amount, after which ultraviolet light cannot be stably obtained.
However, in an ultraviolet generation device used as a light source of, e.g., an exposure device in photolithographic techniques for making a semiconductor device or other various devices or the like, it is desirable to have stability for about 200 hours, and desirable to have at least the half-value period of the output for 200 hours or longer.
The present invention makes it possible to obtain an ultraviolet optical device having an optical part which is subjected to ultraviolet irradiation according to the above-described object.
The present invention makes it possible to obtain an ultraviolet optical device comprising an optical part on which an ultraviolet ray is irradiated, which meets the above-described object.
The present invention is an ultraviolet optical device comprising optical parts on which an ultraviolet ray is irradiated, wherein some or all of the optical parts are arranged in an envelope having a gas inlet port and a gas output port, and a dry gas led from the gas inlet port is sprayed on an optical part, which has a problem of deterioration in its optical characteristics.
The ultraviolet optical device according to the present invention is an ultraviolet optical device, having a nonlinear optical element in a resonator, for generating an ultraviolet ray, wherein the relationship between time T [time] at which an ultraviolet output is x [%] with respect to an initial output and a volume fraction Rw [ppm] of moisture of an arrangement portion of all or some of the optical parts on which the ultraviolet ray is irradiated is expressed by the following expression:
Txe2x89xa7(5xc3x97104xcex3SH0.5PUVxe2x88x920.5 exp(xe2x88x920.00081381xc2x7Rw))xc3x97(xe2x88x92Pi+PUV+xxe2x88x920.5(Pixe2x88x92PUV/2))xe2x80x83xe2x80x83(1)
where, Pi, PUV, and xcex3SH are constants,
Pi is an output [W] of an incident fundamental wave onto the nonlinear optical element,
PUVis an ultraviolet output [W] on an emission end face of the nonlinear optical element, and
xcex3SH is a nonlinear conversion factor [Wxe2x88x921]).
In addition, the ultraviolet optical device according to the present invention is an ultraviolet optical device, having a nonlinear optical element in a resonator, for generating an ultraviolet ray, wherein the relationship between an increase in in-resonator loss xcex94xcex4/xcex94T [%/hour] per unit time and a volume fraction Rw [ppm] of moisture of an arrangement portion of all or some of the optical parts on which the ultraviolet ray is irradiated is expressed by the following expression:
xcex94xcex4/xcex94Txe2x89xa62xc3x9710xe2x88x925 exp(xe2x88x920.00081381xc2x7Rw)xe2x80x83xe2x80x83(3).
As described above, according to the present invention, it is confirmed that the deterioration in optical characteristics of an optical part on which an ultraviolet ray is irradiated can be considerably reduced by actively spraying the dry gas.
In each of the devices according to the present invention, an amount of moisture in the arrangement portion of the optical part on which the ultraviolet ray is irradiated is selected to reduce an ultraviolet output, i.e., to set the life time of the optical part.